1. Field of the Invention
This invention relates to a starting circuit for an inverter, and more particularly to a starting circuit for an inverter by which it can be certain that the inverter is started.
2. Description of the Prior Art
A DC-DC converter including a starting circuit 10, as shown in FIG. 1, is well known. In FIG. 1, an AC voltage of a predetermined level is applied to a pair of input terminals 1a and 1b, and it is rectified by a rectifying circuit 2 which is constituted, for example, by a diode-bridge. A positive voltage is led out in a line 4a, while a negative voltage is led out in another line 4b. Smoothing capacitors C.sub.1 and C.sub.2 are connected in series with each other between the lines 4a and 4b. The line 4a is connected to one end of a parallel circuit of a capacitor C.sub.5 and a resistor R.sub.7, to the anode side of a thyristor 8 and to the collector electrode of a transistor Q.sub.1.
The other end of the parallel circuit of the capacitor C.sub.5 and the resistor R.sub.7 is connected to the gate terminal of the thyristor 8. The cathode side of the thyristor 8 is connected through a parallel circuit of a current limiting resistor R.sub.8 and a speed-up capacitor C.sub.6 to the base electrode of the transistor Q.sub.2. The collector electrode of the transistor Q.sub.2 and the emitter electrode of the transistor Q.sub.1 are connected to each other, and they are connected to one end of the secondary winding 11c of an input transformer 11 having a saturable magnetic core, and to one end of another secondary winding 11b of the input transformer 11. The other end of the secondary winding 11b is connected to the base electrode of the transistor Q.sub.1. A further secondary winding 11d is connected to the transistor Q.sub.2. The secondary windings 11b and 11d are wound round the saturable magnetic core of the input transformer 11 in opposite directions or in opposite polarities to each other. The secondary winding 11c is used for positive feedback, and so it works to increase voltages induced at the secondary windings 11b and 11d so as to compensate the base voltages of the transistors Q.sub.1 and Q.sub.2 when the inverter is highly loaded. The other end of the secondary winding 11c is connected through the primary winding 5a of the output transformer 5 to the connection point between the capacitors C.sub.1 and C.sub.2. The base electrode of the transistor Q.sub.2 is connected to one end of the secondary winding 11d of the input transformer 11. The other end of the secondary winding 11d is connected to the emitter electrode of the transistor Q.sub.2 and to the line 4b. Accordingly, the transistor Q.sub.2 is put into the conductive state initially with the current supplied to the base electrode of the transistor Q.sub.2 through the thyristor 8, and then with the voltage induced at the secondary winding 11d.
Both ends of a secondary winding 5b of the output transformer 5 are connected to a rectifying circuit 12 formed by a diode bridge to apply the output voltage of the output transformer 5 to the rectifying circuit 12. The output voltage is rectified by the rectifying circuit 12, so that a DC voltage of a predetermined level is obtained from output terminals 14a and 14b. Another secondary winding 5c of the output transformer 5, as a feedback winding, is connected through a resistor R.sub.6 to the primary winding 11a of the input transformer 11. Thus, a part of the output obtained from the output transformer 5 is fed back through the resistor R.sub.6 to switch on and off alternately the transistors Q.sub.1 and Q.sub.2.
Next operation of the above described circuit will be described.
With the application of the AC voltage to the input terminals 1a and 1b of the rectifying circuit 2, a charging current flows into the capacitor C.sub.5 from the line 4a to put the thyristor 8 into the conductive state. Accordingly, a current from the line 4a is supplied through the thyristor 8 and the capacitor C.sub.6 to the base electrode of the transistor Q.sub.2 to put the latter into the conductive state. Accordingly, as the capacitor C.sub.6 is charged, namely the current flowing through the capacitor C.sub.6 decreases, the current flowing through the resistor R.sub.8 increases. Thus, a collector current of the transistor Q.sub.2 flows from the capacitor C.sub.2 to the line 4b through the primary winding 5a of the output transformer 5, the secondary winding 11c of the input transformer 11, and the transistor Q.sub.2. As a result, an output voltage having a width corresponding to the time for which the transistor Q.sub.2 is put in the conductive state, is obtained from the secondary winding 5b of the output transformer 5.
When the output voltage is obtained from the one secondary winding 5b of the output transformer 5, another output voltage is obtained from the other seecondary winding 5c of the output transformer 5, and it is applied through the resistor R.sub.6 to the primary winding 11a of the input tranformer 11. Accordingly, secondary voltages are induced at the secondary windings 11b and 11d in opposite polarities to each other. With the application of the induced voltage of the secondary winding 11d, the transistor Q.sub.2 continues to be put into the conductive state. And with the increase of the voltage applied to the primary winding 11a, the magnetic core of the input transformer 11 is saturated to put the induced voltages of the secondary windings 11b and 11d nearly into zero, so that the transistor Q.sub.2 is suddenly put into the non-conductive state. The current to the primary winding 5a of the output transformer 5 from the capacitor C.sub.2 is suddenly interrupted. Thus, a higher voltage is induced at the secondary winding 5c in the reverse polarity to the previously induced voltage, and it is applied to the primary winding 11a of the input transformer 11. The magnetic core of the input transformer 11 becomes unsaturated, and so voltages are induced at the secondary windings 11b and 11d in the reverse polarities to the previously induced voltages, respectively. As a result, the transistor Q.sub.1 is now put into the conductive state with the application of the induced voltage of the secondary winding 11b to the base electrode of the transistor Q.sub.1. A current now flows from the line 4a to the capacitor C.sub.1 through the transistor Q.sub.1, the secondary winding 11c and the primary winding 5a of the output transformer 5. The output is obtained from the secondary winding 5b of the output transformer 5. The other output from the secondary winding 5c is applied to the primary winding 11a of the input transformer 11. As above described, the magnetic core of the input transformer 11 is again saturated to suddenly put the transistor Q.sub.1 into the non-conductive state. And the transistor Q.sub.2 is again put into the conductive state. Thus, the transistors Q.sub.1 and Q.sub.2 are alernately put into the conductive or non-conductive state. A continuous pulse output of a predetermined frequency and level is obtained from the secondary winding 5b of the output transformer 5. The continuous pulse output is rectified by the rectifying circuit 12. A DC voltage of a predetermined level is obtained from the output terminals 14a and 14b, and it is supplied to a load circuit (not shown).
However, in the above described circuit, the starting operation by the thyristor 8 is very unstable, and the switching operation of the transistors cannot often be smoothly effected.
A part of the current flowing through the thyristor 8 is shunt to the secondary winding 11d of the input transformer 11. Accordingly, the base current to the transistor Q.sub.2 is not sufficient enough to put the transistor Q.sub.2 into the conductive state, in some cases. Thus, the starting operation is very unstable. Even when the switching operation of the transistors Q.sub.1 and Q.sub.2 is smoothly started, the conduction and non-conduction of the thyristor 8 are repeated in accordance with the time constant of the parallel circuit of the capacitor C.sub.6 and resistor R.sub.8, and so the current flowing through the parallel circuit becomes load or loss in operation.